This invention relates to a fluid pumping system and, more particularly, to a fluid pumping system adapted for use with a natural gas dehydrating system of the type employed at a gas well head to remove water from a well stream composed of a mixture of gas, oil and water.
Examples of such gas dehydrating systems are disclosed in U.S. Pat. Nos. 3,094,574; 3,288,448; 3,541,763; 4,402,652 and U.S. Pat. application Ser. No. 661,398 filed Oct. 16, 1984, now U.S. Pat. No. 4,588,434,by Charles Richard Gerlach and Rodney Thomas Heath; the disclosures of which are specifically incorporated herein by reference. In general, such systems comprise a separator means for receiving the oil and water liquids from "wet" (water vapor laden) gas; and a water absorber means, which employs a liquid dehydrating agent such as glycol, for removing the water vapor from the wet gas and producing "dry" gas suitable for commercial usage. The glycol is continuously supplied to the absorber means in a "dry" low water vapor pressure condition and is removed from the absorber means in a "wet" high water vapor pressure condition. The wet glycol is continuously removed from the absorber means and circulated through a reboiler means, which includes a still column, for removing the absorbed water from the glycol and heating the glycol to provide a new supply of hot dry glycol. Heating of the glycol in the reboiler means is generally accomplished through use of a gas burner mounted in a fire tube. The hot dry glycol from the reboiler means passes through a heat exchanger, where the hot dry glycol transfers some of its heat to incoming wet glycol going to the still column. The dry glycol subsequently passes to a dry glycol storage tank. A glycol passage means is provided to enable passage of wet glycol from the absorber means to the reboiler means and to pump dry glycol from the storage tank to the absorber means.
As described in U.S. Pat. Nos. 4,286,929 and 4,402,652, the disclosure of which is hereby incorporated herein by reference, motors for glycol pumps of natural gas dehydrating systems have heretofore been designed to be operated by the energy of natural gas available at the well head due to the relatively high pressures and temperatures thereof. In addition, the energy of the wet glycol has been used to drive a single piston pump for the dry glycol as disclosed in U.S. Pat. No. 3,093,122 to Sachnik dated June 11, 1963. This pumping unit uses a fluid driven power piston, and a pilot valve driven by the same fluid controls the rate of operation of the master slide valve, which distributes fluid to the piston pump.
One of the problems with pump designs has been that the pressure of the gas stream from natural gas wells is highly variable and pumps have often required large amounts of energy. Furthermore, changes in gas pressures during day to day operation have often caused stalling of the pump and interruption of the entire dehydrating system. Since the dehydrating systems are continuously operated at the well site without continuous monitoring by operating personnel, reliable continuous operation of the pump is of critical importance.
The pumping units described in U.S. Pat. No. 4,286,929; U.S. Pat. No. 4,402,652 and U.S. Pat. application, Ser. No. 661,398 filed Oct. 16, 1984, now U.S. Pat. No. 4,588,434 of Charles Gerlach and Rodney Heath (all of which are hereby specifically incorporated by reference for all that is disclosed therein) have proven to be highly efficient and require only relatively small amounts of gas to drive the pumps. Such pumps are less subject to stalling at lower operating speeds than earlier pumps and are effective over a relatively wide range of operating conditions. However, problems still occur when gas flow drops to zero or low flow rates as a result of high line pressure, stopcocking, line freezes, etc. During these no-flow or extremely low flow conditions, energy is consumed by the pump to circulate glycol and energy is also consumed by the burner to heat the circulating glycol. Since little or no gas to glycol heat exchange occurs under these conditions, the glycol and associated containment vessels can reach elevated temperatures. The results of such overheating are premature pump and packing failures as well as loss of glycol through the still column and unnecessary gas consumption. Further, when gas flow is reestablished, it may take several hours for the unit to cool sufficiently to operate effectively. One solution to this problem, discussed in U.S. Pat. No. 4,402,652, is to install a pump shut down device to terminate pumping operations under gas no-flow conditions. An orifice meter is located in the well gas line and actuates a shut off valve in the supply gas line to terminate pump operation. One drawback is that pump start-up subsequent to a termination of operations may be unreliable. Another problem is that such a system, in some situations, does not provide an appropriate response to fluctuating extreme conditions. For example, if the well gas flow were only briefly interrupted it would be undesirable to terminate pumping and, on the other hand, a substantially reduced well gas flow rate which might produce elevated heating conditions after a relatively long period of time might not be of a sufficiently low flow rate to trigger the pump shut off mechanism.
Thus, a need exists for providing a control system which is capable of preventing overheating of a glycol type dehydration system under certain conditions associated with reduced gas flows and which provide an appropriate response to such conditions in all situations to provide optimum system operation.